Molecular atomic clock with wave propagating rotational spectroscopy cell

ABSTRACT

A clock apparatus with: (i) a gas cell, including a continuous path cavity including a sealed interior for providing a signal waveguide; (ii) an apparatus for providing an electromagnetic wave to travel along the continuous path cavity and for circulating around the continuous path cavity back toward and past a point of entry of the electromagnetic wave in the continuous path cavity; (iii) a dipolar gas inside the sealed interior of the cavity; and (iv) receiving apparatus for detecting an amount of energy in the electromagnetic wave, wherein the amount of energy is responsive to an amount of absorption of the electromagnetic wave as the electromagnetic wave passes through the dipolar gas.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to, the benefit of the filing date of,and hereby incorporates herein by reference: U.S. Provisional PatentApplication No. 62/612,095, entitled “Molecular Atomic Clock With WavePropagating Rotational Spectroscopy Cell,” filed Dec. 29, 2017.

This application is related to and U.S. patent application Ser. No.16/234,492, filed on even date, and entitled, “Molecular Atomic ClockWith Wave Propagating Rotational Spectroscopy Cell.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND

The example embodiments relate to precision frequency clock signalsgenerated from molecular rotational quantum response in a cell and, moreparticularly, to a system incorporating wave interrogation of the cell.

Precision clock signals, usable as a base frequency source eitherdirectly, or converted (e.g., divided down) to some multiple of a basefrequency source, can be generated from various circuits andconfigurations. One precision clock signal example is an atomic clock,so named as its signal is produced in response to the natural andquantum response of atoms or molecules, to an excitation source. In oneapproach, such atoms are in the form of alkali metals stored in achamber, where the excitation source can be one or more lasers directedto the cell and the response of the chamber atoms is detected bymeasuring the amount of laser energy (photons) that passes through thechamber as the laser frequency sweeps across a range. In anotherapproach, such molecules are in the form of dipolar gases also stored ina chamber, where the excitation source is an electromagnetic wavepropagating through the chamber and the response of the chamber atoms isdetected by measuring the amount of electromagnetic energy that passesthrough the chamber as the energy source sweeps across a range.

Further to the above, an example of a millimeter wave atomic clock isdescribed in U.S. Pat. No. 9,529,334 (“the '334 Patent”), issued Dec.27, 2016, hereby incorporated fully herein by reference, and which isco-assigned to the same assignee as the present application. The '334Patent illustrates, among various other things, an atomic clockapparatus including a sealed cavity storing a dipolar gas, with anelectromagnetic entrance into which an electromagnetic wave (or field)enters near a first end of the cavity and an electromagnetic exit fromwhich an electromagnetic wave exits near a second end of the cavity. Theelectromagnetic wave that so exits is measured to determine an amount ofabsorption by (or transmission through) the dipolar gas, with themeasure indicative of the quantum response of the gas as a function ofthe wave frequency. The '334 Patent also discusses a selection ofpressure for the cavity-sealed dipolar gas, noting that pressurereduction below a desired pressure would reduce the magnitude of thepeak response transition, thereby degrading the ability to detect andtrack the quantum response of the dipolar gas.

While the prior art approaches described above can provide useful toquantum response detection, the present application providesalternatives to the prior art, as further detailed below.

SUMMARY

In an embodiment, there is a clock apparatus with: (i) a gas cell,including a continuous path cavity including a sealed interior forproviding a signal waveguide; (ii) an apparatus for providing anelectromagnetic wave to travel along and circulate around the continuouspath cavity back toward and past a point of entry of the electromagneticwave in the continuous path cavity; (iii) a dipolar gas inside thesealed interior of the continuous path cavity; and (iv) receivingapparatus for detecting an amount of energy in the electromagnetic waveafter the electromagnetic wave passes through the dipolar gas.

Numerous other inventive aspects are also disclosed and claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of an example embodiment clocksystem.

FIG. 2 illustrates a method of operation of the system of FIG. 1.

FIG. 3 illustrates a plot of the RX energy absorption signal in responseto the energies E₁ and E₂ from the pump and probe of the system in FIG.1.

FIG. 4 again illustrates a schematic diagram of the FIG. 1 system withadditional detail to a first example embodiment configuration for thebidirectional coupler.

FIG. 5 illustrates a schematic diagram of an alternative exampleembodiment clock system with pump and probe signals at respectivelongitudinal ends of the cell cavity.

FIG. 6 illustrates a schematic diagram of an alternative exampleembodiment clock system with a reflector at an opposing end of the cellcavity relative to the pump signal entrance to the cavity.

FIG. 7 illustrates a schematic diagram of an alternative exampleembodiment clock system with both the pump signal and receive signalpassages departing away from the longitudinal axis of the cell cavity.

FIG. 8 illustrates a schematic and perspective diagram of an exampleembodiment configuration for a monolithically integrated bidirectionalcoupler and cavity.

FIG. 9 illustrates a schematic diagram of an alternative exampleembodiment with a circulating path and bidirectional travel ofelectromagnetic waves interrogating the dipolar gas.

FIG. 10 illustrates a schematic diagram of an alternative exampleembodiment with a circulating path and unidirectional travel ofelectromagnetic waves interrogating the dipolar gas.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic diagram of an example embodiment clocksystem 10 ₁. Clock system 10 ₁ is understood in general with referenceto three components, a gas cell 12, a bidirectional coupler 14, and atransceiver 16, where ultimately transceiver 16 provides a stablereference clock signal REFCLK in response to bidirectionalelectromagnetic interrogation of a dipolar gas in cell 12. Dipolar gasis preferred, as its electrical dipolar nature provides a detectableresponse to an interrogating electromagnetic wave, as further exploredbelow. Various other additional details about these components,interactions between them, manners of construction, and the like aredetailed in the remainder of this document.

Cell 12 is preferably formed in connection with an integrated circuitwafer, which can include multiple layers affixed relative to asemiconductor substrate (see, e.g., the incorporated by reference U.S.Pat. No. 9,529,334). In the illustration of FIG. 1, as well as invarious subsequent figures, the shape depicted of cell 12 is intended todepict a generally top-down view, such as a planar cross-sectional viewparallel to the plane generally defined by the substrate in which thegas storage cavity of cell 12 is formed. In general, cell 12 includes asealed enclosure having an interior in which a gas is stored. Morespecifically, cell 12 stores a dipolar gas, such as water vapor (H₂O),CH₃CN, HC₃N, OCS, HCN, NH₃, and isotopes of these gases, or any otherdipolar molecular gas, inside an enclosed cavity of the cell, the cavitybeing sealed by nature of shapes, layering, and the like relative to thesemiconductor substrate and layers that combine to enclose the dipolargas at a relatively low (e.g., 0.1 mbar) pressure. For reasons detailedbelow, however, the enclosed pressure can be other than the exampleprovided, as an example embodiment affords additional beneficial resultsthat are independent of, or minimally affected by, the sealed pressureof the dipolar gas. Cell 12 also preferably includes, or is lined alongmost of its interior surfaces with, a material to facilitate theinterior as a signal waveguide, where such material may be, for example,a conductor or a dielectric. Also for reasons evident hereinbelow,preferably cell 12 includes a majority longitudinal length having alinear axis 12 _(AX) and along which electromagnetic waves may travel,where the cross-sectional shape of cell 12 perpendicular to axis 12_(AX) may be square, rectangular, trapezoidal, or still other shapes.The dimensions of cell 12 may vary, where for example it may be 30 to150 mm long, 30 to 50 mm wide, and 0.5 millimeter tall, where selectingthese or comparable sizes are intended to match properties for efficientwave propagation given the frequency of the desired wave. Moreover,while the longitudinal shape is linear in FIG. 1 (and other figures), italso may bend or turn so as to form, for example, a meandering path. Invarious embodiments, cell 12 includes an additional cavity portion thatdeparts away from the linear portion of axis 12 _(AX), where in FIG. 1,this non-coaxial or non-co-linear departure is shown generally at 12_(DP).

Bidirectional coupler 14 provides structure, detailed later, forpropagating electromagnetic waves in both directions along axis 12 _(AX)of cell 12. Accordingly, in the example of FIG. 1, coupler 14 isconnected to a probe antenna ANT_(TXPROBE) and to a pump antennaANT_(TXPUMP). Probe antenna ANT_(TXPROBE) is positioned proximate afirst passage 12 _(P1) of cell 12, so that electromagnetic energy fromcoupler 14 may be communicated to probe antenna ANT_(TXPROBE) and theninto first passage 12 _(P1) as an entrance into cell 12 and thereby topass in general in a first direction D₁ shown by a dashed arrow inFIG. 1. Note that the term passage in the context of first passage 12_(P1) is intended to suggest a signal communications pathway for passageof the electromagnetic signal, but not an open aperture to ambient perse that otherwise could cause the sealed dipolar gas in cell 12 toescape; accordingly, such a pathway may be formed in various fashions,such as by a glass layer as the upper surface of the sealed enclosure ofthe cell and providing an opening in the metal surround that isotherwise formed within the cell—in this manner, an electromagneticsignal may pass through the opening and glass into the interior of thecell, thereby reaching the dipolar gas sealed therein. Pump antennaANT_(TXPUMP) is positioned proximate a second passage 12 _(P2) of cell12, so that electromagnetic energy from coupler 14 may be communicatedto pump antenna ANT_(TXPUMP) and then into second passage 12 _(P2) as anentrance into cell 12 and thereby to pass in general in a seconddirection D₂, also shown by a dashed arrow in FIG. 1. Accordingly, for amajority 12 _(M) of the length along axis 12 _(AX), the signalscommunicated by coupler 14 provide bidirectional interrogation of thedipolar gas in cell 12.

Transceiver 16, as its name suggests, is both for transmitting (TX) andreceiving (RX) signals. Internally within transceiver 16 are illustratedfour blocks 18, 20, 22, and 24, by way of example but not limitation,for accomplishing the transceiver and signal responsiveness describedherein. In this regard, a signal generator 18 is connected, and ismodulated by a modulator 20, to provide a base frequency controlled TXsignal that, as detailed later, is swept across a particular frequencyrange from below to past the intrinsic quantum rotational statetransition frequency for the dipolar gas in cell 12 (e.g., 183.31 GHzfor water). Modulator 20 modulates the frequency of the interrogationsignal provided by the signal generator 18. The modulation frequencyranges, for example, between 10 to 50 KHz. After the signal passesthrough the gas cell, it is received by a lock in amplifier 22, via aninput 16 _(IN) coupled to receive the RX signal from a receive antennaANT_(RX). Particularly, antenna ANT_(RX) is positioned proximate a thirdpassage 12 _(P3) of cell 12, so that electromagnetic energy from alongaxis 12 _(AX) may be communicated from passage 12 _(P3), as an exit, toreceive antenna ANT_(RX) and then to transceiver 16 and, moreparticularly, to lock in amplifier 22. In general, lock in amplifier 22uses the signal from the modulator 20 to measure the RX signal at thesame modulation frequency provided by modulator 20. In this way, lock inamplifier 22 is able to reject noise outside the modulation frequencyand thereby reduce the noise from the system.

In more detail, the TX signal may be a sinusoid, although other periodicoscillating signals also may be used, so long as such signal includes aFourier component in the frequency of interest. The TX signal isconnected, via an output 16 _(OUT), to bidirectional coupler 14. Underfeedback control, signal generator 18 also provides theultimately-refined reference clock REFCLK. The RX signal represents anamount of the originally transmitted signal TX that passes through cell12 and contains the information of the absorption of the dipolar gas atthe quantum rotations transition frequency. In response, lock inamplifier 22 provides a signal that is the first derivative of thesignal as it is swept in frequency. Consequently at the frequencycorresponding to the quantum rotational molecular transition, the firstderivative is zero and the error signal ERR is zero. At frequenciesdifferent from the quantum rotational transition, the signal ERR is notzero and provides a correction to the loop filter 24, allowing it to“lock” the clock to the quantum transition frequency. This apparatusalso filters out noise as detected by reference to the modulationfrequency provided by modulator 20. In one example, lock in amplifier 22provides the error signal ERR as an in-phase output, and the errorsignal ERR is used as an input by a loop filter 24 (or controllercircuit) for providing a control output signal CO to signal generator18. As further detailed below, such feedback selectively adjusts the TXoutput signal frequency, following an initial sweep, to ultimatelymaintain this frequency at a peak absorption frequency of the dipolarmolecular gas inside the sealed interior of cell 12, with thatmaintained frequency providing a stable output reference clock REFCLK.In some examples, the RF power of the TX and RX loop is controlled so asto avoid or mitigate stark shift effects (frequency shifts in responseto quantum transition produced by the presence of an electric field).

The overall operation of system 10 ₁ is now described, in connectionwith the flow chart method 30 of FIG. 2. By way of introduction, notethat method 30 is shown as a sequential flow process for sake ofdiscussion, while in general the steps provide a feedback control and,accordingly, each step may actually occur in a time overlapping one ormore of the other steps. In a step 32, transceiver 16 sweeps themodulated base frequency TX signal such that the base frequency is sweptacross an initial frequency range FR that is intended to include theintrinsic quantum rotational state transition frequency for the dipolargas in cell 12. Thus, in the example where the dipolar gas is water, therange will include the intrinsic quantum rotational state transitionfrequency of 183.31 GHz for water, and could include, for example, asweep from 183.28 GHz to 183.34 GHz. Thus, the TX signal delivers anenergy E to bidirectional coupler 14, sweeping across this frequencyrange, so that the same frequency is simultaneously applied by bothantennas ANT_(TXPUMP) and ANT_(TXPROBE), so as to achieve thebidirectional propagation along directions D₁ and D₂ introduced above.In addition, while the TX signal represents a certain amount of energyE, preferably coupler 14 couples a first amount E₁ of that energy toantenna ANT_(TXPROBE) and a second amount E₂ of that energy (i.e.,subject to possible signal loss TX−E₁=E₂) to antenna ANT_(TXPUMP).Preferably E₂>E₁, where for example E₂ may be 90% of TX, leaving 10% ofTX as E₁.

In step 34, transceiver 16 converges to, or locks in at, a Doppler freefrequency ƒ_(T) at which peak absorption of the electromagnetic waveoccurs in the dipolar gas sealed in cell 12, in response to the sweepingfrequency from step 32 and the corresponding detected response.Particularly, steps 32 and 34 may essentially occur simultaneously, asduring step 34, receive antenna ANT_(RX) couples the RX signal, whichoccurs from the TX signal of step 32, to lock in amplifier 22. Lock inamplifier 22 also receives the modulation frequency from modulator 20,so that according to known lock in methodology, noise may be reducedfrom the received signal so as to detect the received energycorresponding to the base frequency TX signal that is input to cell 12from signal generator 18. Further, lock in amplifier 22 (or associatedcircuitry) evaluates a derivative of the RX signal as the step 32 sweepoccurs. Accordingly, as the RX signal is changing (in response to aquantum response in cell 12), the derivative is non-zero, whereas as theRX signal reaches a peak (maximum or minimum) the derivative approachesor is zero. This derivative determination provides an error signal ERR,connected as an output to loop filter 24, and as further explained belowthe error ERR signal is expected to reach zero at three different TXbase frequencies.

In step 36, loop filter 24 provides a control signal CO to signalgenerator 18, so as to refine control of signal generator 18 to makeadjustments in the frequency of TX to restrain or refine that frequencyin a feedback look controlled manner to thereafter maintain the basefrequency of TX at the center frequency among the three different TXbase frequencies for which the error signal ERR is zero. This centerfrequency represents the frequency ƒ_(T) at which peak absorption occursof the electromagnetic wave occurs in the cell dipolar gas. In thisregard, note that loop filter 24 is preferably a proportional integralderivative (PID) circuit, with an output that sums values, multipliedtimes respective controlled parameters, corresponding to the threefactors (i.e., proportional, integral, derivative) that give the PIDcircuit its name. Accordingly, the output of loop filter 24 facilitatesa clock signal with good stability and reduces or eliminates thepossibility of the signal generator creating a signal that oscillatesaround the around desired frequency. Accordingly, at this point, the TXsignal should maintain a base frequency at or near the intrinsictransitional quantum frequency of the dipolar gas, and that same basefrequency is output as the reference clock signal REFCLK, as may be usedby other circuitry requiring a frequency-precise clock signal. Indeed,method 30 may be regarded as an ongoing loop running at selected (orall) times that the clock is operational and endeavoring to lock theclock at most or all instants where, once locked after an initial scanof steps 32 and 34, the loop continues to perform across a secondfrequency range that is smaller in bandwidth than the above-describedstep 32 initial frequency range FR, where the second frequency range isexpected to still include the center frequency representing frequencyƒ_(T). In other words, the step 32 sweep and step 34 response detectioncan essentially repeat in step 36, but using a narrower bandwidth formore efficient refinement or maintenance of the base frequency at thepeak absorption frequency ƒ_(T), as further illustrated hereinbelow.

Given the preceding, one skilled in the art will appreciate that system101 facilitates electromagnetic interrogation of the dipolar gas withincell 12, by transmitting TX at varying (e.g., sweeping) frequencieswithin a defined band around a suspected quantum absorption frequency atwhich the transmission efficiency of cell 12 is minimal (absorption ismaximal), and when the system detects a null or minima in transmissionenergy (or maximum in absorption), the TX output signal frequency isregulated to operate at the frequency so detected, thereby in responseto the natural quantum behavior of the dipole. As a result, REFCLK, likethat quantum behavior, is generally stable with respect to time (doesnot degrade or drift over time) and is largely independent oftemperature and a number of other variables.

Having introduced system 10 ₁ and certain of its components, certainobservations are now provided in connection with the prior art and forcontrast with the example embodiment benefits. First by way ofintroduction, the Allan deviation (which is the square root of the Allanvariance) is a measure of frequency stability of clock signals (e.g., inclocks, oscillators, amplifiers), and in effect is a measure offrequency distribution over a number of samples—hence, the lower theAllan deviation measure, the lower the variance of distribution and thebetter the performance of the clock signal. Further, the Allan deviationis inversely proportional to both the quality factor (Q) andsignal-to-noise ratio (SNR) of the frequency response curve, whether inthe prior art, or transceiver 16 in an example embodiment, as itprocesses the received RX signal response to detect an absorption peakfrequency; thus, by increasing one or both of the Q and SNR of thesystem, the Allan deviation measure is improved. Notably, the prior artcan achieve certain improvement in the Allan deviation by reducingpressure within the cell which reduces the width of the transitioncaused by pressure broadening phenomena, as such an approach willimprove Q only down to a certain pressure; however, for pressures belowa certain value (e.g., 0.1 mbar), the width cannot be reduced furtherbecause of Doppler broadening which is independent of pressure andmostly dependent on the temperature of the gas. However pressurereduction reduces the number of molecules available for interrogationand so reducing the amplitude of the quantum transition signal. Insummary an optimum pressure can be found (for water this pressure isapproximately 0.1 mbar) where the SNR can be maximized by having theminimum transition width possible and the maximum amplitude possible.

As mentioned above, the prior art endeavors can be susceptible toDoppler broadening, which is an effect caused by the distribution ofvelocities of atoms/molecules, particularly in response to highertemperatures. Particularly, for atoms having velocities in a samedirection as the direction of propagation of the electromagnetic wave,the Doppler effect will cause the transition of the dipolar moleculesfrom the lower energy vibrational state to the higher energy vibrationalstate to occur when the frequency of the electromagnetic signal is lowerthan the frequency that corresponds to the energy difference of the twovibrational states (E=h f) (e.g., 183.31 GHz for water), and conversely,for atoms having velocities in an opposing direction as the prior artunidirectional signal, the Doppler effect will cause the rotationaltransition of those atoms to occur at an excitation frequency that ishigher than the intrinsic quantum rotational state transition frequencyfor the dipole. Thus, the result of Doppler spreading is a widerspectral line when interrogating these atoms/molecules, whichaccordingly provides less accuracy in identifying a particular frequencyof quantum transition.

Reference is now turned to the contrast of example embodiment benefits,particularly in connection with the prior art observations above and theexample embodiment implementation of bidirectional signal paths alongmajority 12 _(M) of cell 12, as further demonstrated by the plot of FIG.3. Specifically, FIG. 3 illustrates a plot of the RX energy absorptionby the gas in cell 12, in response to the energies E₁ and E₂ from thepump and probe TX signals, respectively, and as a function of frequency.Recall both the probe and pump are at the same frequency, and thatfrequency is swept across a range that is intended to include theintrinsic quantum rotational state transition frequency ƒ_(T) for thedipole (e.g., 183.31 GHz for water). In general, therefore, the sweepingof frequency may be low to high (or high to low), producing the responseshape depicted in FIG. 3. Note that while FIG. 3 plots absorption,transmission energy is provided in the signal TX and detected in thesignal RX, so a plot, normalized to a value of one, is essentially oneminus the signal plot of FIG. 3. In any event, as the swept frequencyapproaches the intrinsic quantum rotational state transition frequency,the absorption of energy by the dipolar gas increases as shown (or theamount of energy transmitted through the cell decreases), and asdetected in the RX signal, creates an absorption spectra that is flatteraway from that intrinsic frequency and that ascends from both directionsas the frequency sweep nears the dipolar gas intrinsic frequency.Additionally, as the intrinsic frequency is approached, a first peak PK₁occurs which is shown as a maximum in terms of energy absorption and ata frequency below the Doppler free frequency ƒ_(T), and similarly aboveƒ_(T) a second peak PK₂ occurs. As further detailed below, however,example embodiments are able to detect an additional peak as the Dopplerfree frequency ƒ_(T), between peaks PK₁ and PK₂.

Further regarding the FIG. 3 response, recall that an example embodimentprovides one of the bidirectional TX signals at a higher energy than theother, where for sake of convention in other technologies thehigher-energized signal is termed the pump. As a result, it isanticipated in connection with some example embodiments that most of theatoms interrogated by the higher-energized pump signal, at theappropriate quantum frequency, will be excited to an energy level higherthan a lower energy (e.g., ground) state, while other of the atoms willremain at the lower energy state. Thus in response to the probe signal,the number of molecules that are in the ground state is significantlyreduced because most of them have been excited to the excited state bythe pump signal causing a decrease in the absorption profile. Inaddition, however, the bidirectional or counter-propagating nature ofthe probe and pump signals also reduces or eliminates the Dopplereffect. Particularly, atoms at zero velocity experience do not have theDoppler effect, and are accordingly affected by the frequency aspect ofboth of the counter-propagating waves, where again the pump beam hasdepopulated a portion of the ground state to the higher energy state. Asa result of the preceding, less of the ground state atoms remain atfrequency ƒ_(T), so there are fewer atoms to absorb the probe energy anda corresponding drop in absorption, where such lack of absorption ofthat probe energy is evident in the resultant RX plot of FIG. 3, whichincludes a dip DP centered around at the intrinsic frequency ƒ_(T), thedip arising from the fewer low-energy atoms to absorb the probe signal.Thus, the example embodiment provides so-called Doppler-freespectroscopy, in that Doppler broadening is no longer an issue underthis approach and the benefit in some embodiments may be independent ofthe gas pressure in cell 12. Note further that the frequency widthΔƒ_(DP), between peaks PK₁ and PK₂, DP is considerably less than thefrequency width Δƒ_(O) of the outer Gaussian ascending portions of theplot; hence, the Q relative to the frequency width Δƒ_(DP) isconsiderably better than the Q relative to the frequency width Δƒ_(O) ofthe outer ascending portions. Accordingly, the improved Q of the Dopplerfree architecture improves the Allan deviation of system 10 (and theother comparable systems described in this document).

FIG. 4 again illustrates a schematic diagram of system 10 ₁, withadditional detail provided for one example embodiment configuration forbidirectional coupler 14. While FIG. 4 illustrates blocks in certainaspects, the top-down spatial relationship may be illustrative of anexample embodiment implementation in general in that cell 12, coupler14, and transceiver 16 may be positioned relative to one another, or atleast proximate one another, in connection with a semiconductorsubstrate and/or wafer. For example, cell 12 may be formed as part oflayering affixed to, or part of, the semiconductor substrate, with anupper layer thereof having passages (not shown, but see FIG. 1, passages12 _(P1), 12 _(P1), and 12 _(P3)) being penetrable by electromagneticwaves, such as by way of a glass layer having a conductive layeradjacent thereto and with apertures, corresponding to the passages,formed in the conductive layer. Further, coupler 14 is then formedproximate (e.g., within 5 millimeters) cell 12, being located as closeas possible to reduce losses of the signal traveling from thedirectional coupler and cell 12. Coupler 14 may be formed by members(e.g., conductors, described below) applied (e.g., surface plated) to anupper surface of the above-mentioned glass layer that encloses thedipole-retaining sealed cavity of cell 12. Alternatively and as shownlater in FIG. 8, coupler 14 may be coplanar with cell 12 ormonolithically integrated into the same cavity that forms cell 12.Lastly, transceiver 16 can be formed as a separate integrated circuit,which is then conductively and mechanically affixed nearby coupler 14and cell 12; indeed, while not shown, transceiver 16 may be spatiallyaffixed above either or both of cell 12 and coupler 14, via wirebonding, flip chip, or other packaging techniques.

Turning now in more detail to coupler 14 in FIG. 4, in an exampleembodiment, illustrated in FIG. 4, coupler 14 includes two conductors 14₁ and 14 ₂, which preferably have like dimensions and representmirror-image symmetric structures about an imaginary linear axis 14_(AX), but the two are otherwise physically isolated from one another.For example, each of conductors 14 ₁ and 14 ₂ may be formed byfabrication techniques which, by way of example, may include processesfor patterning and affixing metallic surfaces atop a layer. For example,the layer may be glass, with a common ground metallization below theentirety of coupler 14. Further, such formation facilitates fabricationas well as signal coupling between the conductors. Particularly, thefull 100% energy of the TX signal from transceiver is preferablyconnected to a first end 14 _(2E1) of conductor 14 ₂, while a second end14 _(2E2) of conductor 14 ₂ is connected to the pump antennaANT_(TXPUMP). Meanwhile, a low voltage signal, such as ground, isconnected to a first end 14 _(1E1) of conductor 14 ₁, while a second end14 _(1E2) of conductor 14 ₂ is connected to the probe antennaANT_(TXPROBE). In this configuration, a portion of the TX signal energyis coupled, for example capacitively, inductively, or through othermanner, from conductor 14 ₂ to conductor 14 ₁, where preferably thoseconductors are dimensioned, positioned, and otherwise coupled so thatapproximately 90% of the energy of the TX signal remains in conductor 14₂, while 10% of the energy of the TX signal is coupled to conductor 14₂. Thus, consistent with earlier discussions, the pump antennaANT_(TXPUMP) provides an energy E₂ of 90% of the TX signal in a firstdirection through cell 12, while the probe antenna ANT_(TXPROBE)provides an energy E1 of 10% of the TX signal in a second direction,opposite the first, through cell 12. Lastly in connection with theconstruction of coupler 14, note that preferably the linear distancesalong which the pump and probe signals are to propagate are allpreferably integer multiples of ½ of the wavelength of the RF signal tobe propagated through cell 12. For example, FIG. 4 illustratesdimensions DM₁ and DM₂, as linear structural paths for wave propagation,that is, each linear path is a segment of the total propagation path,where the segment is linear for the respective path. Such dimensions arelikewise preferable for other like linear paths in coupler 14, namely,dimensioned in accordance with this stated proportionality relative tothe RF signal wavelength.

FIG. 5 illustrates a schematic diagram of an alternative exampleembodiment clock system 10 ₂, which in various ways should be understoodby one skilled in the art given the earlier discussion, and the readeris assumed familiar with that discussion (also, transceiver 16 is shownin a simplified view). Where system 10 ₂ differs from system 10 ₁ is thepositioning of certain antennas relative to the second and thirdpassages 12 _(P2) and 12 _(P3) of cell 12. Particularly, in system 10 ₂,while the probe antenna ANT_(TXPROBE) remains proximate first passage 12_(P1), the other two antennas are switched in location, that is, the RXantenna ANT_(RX) is proximate second passage 12 _(P2) and the pumpantenna ANT_(TXPUMP) is proximate third passage 12 _(P3). In thismanner, the bidirectional or counter-propagating signals will pass alongthe entirety 12 _(E) of axis 12 _(AX), as opposed to only a majorityportion 12 _(M) thereof as was the case for system 10 ₁. However, forsystem 10 ₂, the receive antenna passage (12 _(P2)) is no longerco-linear with axis 12 _(AX), but instead the signal is received andcommunicated to transceiver 16 from the non-axial departure 12 _(DP)portion of cell 12.

FIG. 6 illustrates a schematic diagram of an alternative exampleembodiment clock system 10 ₃, which again shares various attributes withearlier embodiments. Where system 10 ₃ differs from system 10 ₂,however, is the elimination of an external probe transmit antenna (see,e.g., ANT_(TXPROBE) in systems 10 ₁ and 10 ₂) and of its respectivepassage 12 _(P1) (see, FIGS. 1 and 5), such that electromagnetic wavesare communicated into cell 12 by only a single transmit antennaANT_(TX), so system 10 ₂ does not include two separate, albeit differentenergy level, transmitters and corresponding transmission passages intothe cell. Instead, transceiver 16 directly couples a TX signal, withoutan intermediate coupler (see, e.g., coupler 14 of FIG. 1), to a transmitantenna ANT_(TX). Further, the single transmit antenna ANT_(TX) isproximate a passage 12 _(P3) of cell 12. Further, while cell 12 stillincludes a passage 12 _(P3) at a first longitudinal end of the linearpathway in the cell, at an opposing second longitudinal end is locatedan electromagnetic wave reflector 12 _(EWR). Given this difference andthe earlier discussion, note that when the pump antenna ANT_(TXPUMP)transmits energy into passageway 12 _(P3) as an entrance into cell 12,that energy proceeds in direction D₁ from the first longitudinal end ofthe linear pathway toward the second longitudinal end of the linearpathway, where a portion of that energy is reflected toward the firstlongitudinal end of the linear pathway, that is, back along directionD₂. Moreover, in this example embodiment, reflector 12 _(EWR) isstructured (e.g., by material, shape, dimension and the like) so as toreflect a considerably reduced energy E₁ of energy, as compared to thelevel of energy E₂ received from pump antenna ANT_(TXPUMP). Accordingly,and consistent with earlier example embodiments, such reflection may bethat energy E₁ provides approximately 10% of the total energy receivedby cell 12 from the TX signal. The remaining structure and operation ofcell 10 ₃, however, is readily comparable to other aspects alreadydescribed.

FIG. 7 illustrates a schematic diagram of an alternative exampleembodiment clock system 10 ₄, again which in various ways should beunderstood by one skilled in the art given the earlier discussion andwith various commonality with other example embodiments. Where system 10₄ differs is that cell 12 is symmetric about its axis 12 _(AX), in thatit includes a longitudinal cavity across a majority 12 _(M) of itsoverall length, but it has equally shaped and sized departure cavityportions 12 _(DP1) and 12 _(DP2), each with a respective passage 12_(P3) and 12 _(P2), for communicating respective electromagnetic signalsbetween cell 12 and the pump antenna ANT_(TXPUMP) and RX antennaANT_(RX), respectively. The majority length longitudinal cavity supportsan area where the pump and probe signal coexist and where the zerovelocity molecules are detected and contribute to the strength of thesignal. Hence, the longer this section, the larger the signal,contributing to a better SNR.

FIG. 8 illustrates an electrical block and perspective diagram of analternative example embodiment clock system 10 ₅, in which a channel 800is formed as a recessed path in a substrate 802, whereby channel 800includes portions that are comparable in certain respects to cell 12 andcoupler 14 of earlier embodiments. Accordingly, cell 12 and coupler 14are monolithically integrated into a same layer in substrate 802 (e.g.,co-planar), as now described. Particularly, channel 800 includes variousportions, which are generally indicated with brackets so as toillustrate functionality, but in the illustrated example the entirechannel 800 functions as a gas cell, that is, a sealed enclosure forstoring atoms to be interrogated by an electromagnetic wave. In thisregard, while not shown, an additional layer (e.g., glass) is disposedatop an upper surface of substrate 802, thereby enclosing the sealed gaswhile also allowing receipt of a TX signal from transceiver 16 andproviding a path for wave propagation after which the signal provides anRX signal back to transceiver 16. In one portion of channel 800 showngenerally by the bracket and coupler reference of 14, there are twoproximate wave paths 14 _(P1) and 14 _(P2) that are in fluidcommunication with one another, with a sidewall aperture coupler 14 _(C)portion in which a number of apertures are formed between the two paths;in this manner, as TX energy enters channel 800, a first portion (e.g.,pump at 90%) of that energy passes through path 14 _(P1) and theremaining portion (e.g., probe at 10%) of that energy passes through theapertures in sidewall aperture coupler 14 _(C) and then through path 14_(P2). Paths 14 _(P1) and 14 _(P2) continue to, and are also in fluidcommunication with, opposing ends of a meandering cell portion 12 _(P).Accordingly, in cell portion 12 _(P), as was the case in earlierembodiments, bidirectional interrogation occurs as the pump wave passesin a first direction through portion 12 _(P), while the probe wavepasses in a second direction, opposite the first direction, also throughportion 12 _(P), thereby bidirectionally interrogating dipolar gas inportion 12 _(P). Finally, some of the energy of the propagating wave isable to pass from portion 12 _(P), back to transceiver 16, in the formof the RX signal from an end 800 _(E) of channel 800.

FIG. 9 illustrates a schematic diagram of an alternative exampleembodiment clock system 10 ₆, which in various ways should be understoodby one skilled in the art given the earlier discussion, and the readeris assumed familiar with that discussion (again, transceiver 16 is shownin a simplified view). Also in general, system 10 ₆ operates accordingto method 30 shown in FIG. 3. Where system 10 ₆ differs from earlierexample embodiments, however, is that cell 12 includes a continuouschamber portion 12 _(CCP), in which electromagnetic waves may enter cell12 and continue to circulate to return toward, and then again passbeyond, a point at which they entered the chamber, as is now furtherexplored. First, as with system 10 ₁, system 10 ₆ again includes a probeantenna ANT_(TXPROBE) proximate a first passage 12 _(P1), a pump antennaANT_(TXPUMP) proximate a second passage 12 _(P2), and an RX antennaANT_(RX) proximate a third passage 12 _(P3). However, each of thosepassages is in a respective extension portion of cell 12, shown asportions 12 _(EP1), 12 _(EP2), and 12 _(EP3). In an embodiment, each ofextension portions 12 _(EP1), 12 _(EP2), and 12 _(EP3) is in fluidcommunication with continuous chamber portion 12 _(CCP). An alternativeembodiment may maintain the gas solely (or predominantly) in thecontinuous chamber portion 12 _(CCP), but this will require some type ofhermetic window between the extension portions and the circular portion.Such a window would introduce losses in the signal path and potentialundesirable reflections, degrading device performance.

In system 10 ₆, an electromagnetic signal is transmitted fromANT_(TXPROBE), via first passage 12 _(P1) into cell 12, that signalpropagates as a wave along the interior of extension portion 12 _(EP1)in a direction D₁, and it continues into continuous chamber portion 12_(CCP) so that direction D₁ thereby can be characterized ascounterclockwise in FIG. 9. Due to the continuous nature of portion 12_(CCP), however, which in the illustrated example is by way of acircular cross-section when sectioned across a plane along the directionof the circulation around the entire structure, then as the wave passes360 degrees it will reach the point at which it entered into thecircular region, that is, where extension portion 12 _(EP1) communicateswith the circular portion of cell 12, and then continue onward to againcirculate in the counterclockwise direction D₁. Similarly, but in anopposite direction, as an electromagnetic signal is transmitted fromANT_(TXPUMP), via second passage 12 _(P2), into cell 12, that signalalso propagates as a wave along extension portion 12 _(EP2), but in adirection D₂, opposite that of D₁, and thus in chamber continuousportion 12 _(CCP) at which point the direction D₂ is clockwise in FIG.9. Further, again the continuous nature of portion 12 _(CCP),facilitates an ongoing circulation of the wave, so that as it passes 360degrees through cell 12 it will reach the point at which it entered intothe circular region, that is, where extension portion 12 _(EP2)communicates with the circular portion of cell 12, and then continueonward to again circulate in the clockwise direction D₂.

The number of wave circulations facilitated in an example embodiment,may repeat a number of times, effectively providing a wave interrogationof the dipolar gas molecules along an effective length far in excess ofa single path interrogation along the circumference of portion 12CCP.

The number of circulations will be a function primarily of the qualityfactor of the cavity (not to be confused with the quality factor of thequantum absorption). The quality factor of an electromagnetic cavity isdefined by the ratio of energy stored in the resonator to the energysupplied to it. Accordingly, the more efficient (better performing) thewalls of the cavity in communicating the wave through the cavity, thebetter will be the confinement and the better will be the quality factorof the cavity and the larger will be the number of circulations. Thecirculation number could be even in excess of 100 circulations underresonance conditions, as can be confirmed, for example, by evaluatingstrength of a given energy signal in the dipolar gas. Also recognized,however, is that a detrimental effect may occur if the cavity qualityfactor is above a particular threshold. In example embodimentmethodology, therefore, a cavity may be implemented and tested for apower broadening effect, as may be detected, for example, from anincrease in the width of the quantum transition, reducing the transitionquality factor. Such an effect may occur when power applied by thetransmitter is increased, or the intensity of the electric field in thecavity reaches high values due to high quality factor of the cavity.Note that increasing power in this regard could be considered aneffective way to improve device performance so as to increase SNR, butsuch power may concurrently diminish the Q of the response curve. And,recalling that Allan deviation is inversely proportional to both SNR andQ of the frequency response curve, then there is a point of diminishingreturn if SNR is increased by a percentage that exceeds a comparablepercentage decrease in Q that was caused by the SNR increase—in otherwords, optimum performance is achieved in the exemplary embodiments bymaximizing the product of SNR and Q. Hence, in an exemplary embodiment,a cell design may be created and tested at increasing levels of TXenergy, and if it is found that an increased signal energy is causingtoo much power broadening so as to undesirably reduce Q beyond anacceptable level, then that effect may be reduced by diminishing thequality factor of the cell design. Example embodiments to accomplishsuch quality factor reduction may be achieved by making the walls of thecavity less efficient, for example by making adding roughness to thewalls or using higher resistivity metals (e.g., switching from gold toaluminum) Example embodiments include a roughness of 500 Angstroms, butalternatives are contemplated with a roughness greater than 500Angstroms, and up to 2 micrometers.

Given the structure of system 10 ₆, note that as waves circulate as hasbeen described, a portion of the energy will exit the circularcross-sectioned portion of cell 12, via extension portion 12 _(EP3), tothird passage 12 _(P3), and a portion of that energy is therebycommunicated to antenna ANT_(RX) and then be received by transceiver 16,consistent with earlier-described embodiments. However, the nature of acontinuous passage causes the remaining energy of the wave to continueto circulate multiple times through the passage, thereby providingadditional energy opportunities to interrogate the dipolar gas cellswithin cell 12. As a result, the number of molecules interrogated isincreased, thereby improving the SNR performance as compared to asingle-pass interrogation system, and advancing toward near saturationoperation. Moreover, by using a pump and probe architecture, the systemprovides a Doppler free transition.

In another aspect of system 10 ₆, in an example embodiment, the shapeand dimensions of cell 12, and particularly its chamber continuousportion 12 _(CCP), are preferably selected to facilitate resonance ofthe wave passing therethrough and to circulate a number of times. Forexample, FIG. 9 illustrates the circular portion outer diameter 12 _(OD)and inner diameter 12 _(ID). These two dimensions are preferablyselected to achieve a waveguide suitable for the band of interest (e.g.,the WR-5 band of 140-220 GHz). Further, for the specific operatingfrequency, a propagating wave will exhibit a specific guided wavelength,λ_(g), inside the waveguide and that can be different from unguidedwaves, and λ_(g) can be determined experimentally or through computersimulations. Given these parameters, and in order to provide beneficialenergy for interrogating the dipolar gas, in an example embodiment thediameter of chamber continuous portion 12 _(CCP) is dimensionedaccording to the following Equation 1:

$\begin{matrix}{D_{m} = \frac{N\;\lambda_{g}}{\pi}} & {{Equation}\mspace{14mu} 1}\end{matrix}$where,

D_(m) is the middle diameter of portion 12 _(CCP), that is, the distancehalfway between outer diameter 12 _(OD) and inner diameter 12 _(ID)(i.e., D_(m)=(12_(OD)+12_(ID))/2);

N is an integer of 1 or greater; and

λ_(g) is the above-introduced guided wavelength of the wave throughchamber continuous portion 12 _(CCP).

Thus, with Equation 1 providing for a middle diameter D_(m) of cell 12,it is anticipated that as the wave circulates repeatedly an integernumber of times around chamber continuous portion 12 _(CCP) (passing apoint from which it earlier entered portion 12 _(CCP) started orpreviously passed), the wave energy will be constructive in phase, so asto provide beneficial signal performance.

Further in connection with FIG. 9, note that the circular nature (whenconsidered in the plane of the FIG. 9) of cell 12 is by way of example,and other continuous paths that permit a wave to circulate as describedalso are contemplated. For example, the continuous path may lie along anoval or a generic polygon. Also in this regard, Equation 1 defines adiameter D_(m) as it relates to a circular perimeter for the continuouspath, given that a diameter of a circle is its perimeter divided by pi.For the more general case of a continuous path having a perimeter P,Equation 1 may be rewritten as the following Equation 2:P=Nλ_(g)  Equation 2Accordingly, for example embodiments with a chamber continuous portionthat is non-circular, Equation 2 sets forth a preferred perimeter forthat portion. Lastly in connection with FIG. 9, the cross-sectionalshape of continuous path 12 _(CCP), taken perpendicular to the directionof wave propagation, also may be selected from various shapes. Forexample, the cross-sectional shape of cell 12 in FIG. 9, perpendicularto direction D₁ (or D₂) may be square, rectangular, trapezoidal, orstill other shapes.

FIG. 10 illustrates a schematic diagram of an alternative exampleembodiment clock system 10 ₇, which in various respects is comparable tosystem 10 ₆ of FIG. 9 and, accordingly, should be understood by oneskilled in the art given the earlier discussion. Also in general, system10 ₇ operates according to method 30 shown in FIG. 3. Where system 10 ₇differs from earlier example embodiments, however, is thatelectromagnetic waves are communicated into cell 12 by only a singletransmit antenna ANT_(TX), so system 10 ₇ does not include two separate,albeit different energy level, transmitters and correspondingtransmission passages into the cell. Instead, transceiver 16 directlycouples a TX signal, without an intermediate coupler (see, e.g., coupler14 of FIG. 1), to a transmit antenna ANT_(TX). Further, the singletransmit antenna ANT_(TX) is proximate a passage 12 _(P1) that islocated in an extension portion 12 _(EP1) of cell 12, where theextension portion, as was the case with system 10 ₆ of FIG. 9, fluidlycommunicates with a continuous chamber portion 12 _(CCP). Portion 12_(CCP) again is preferably circular in cross-section taken along a planeof wave travel and has a middle diameter D_(m), as may be establishedpursuant to the above-described Equation 1. As a result, anelectromagnetic signal transmitted from antenna ANT_(TX), via passage 12_(P1) into cell 12, propagates as a wave along the interior of extensionportion 12 _(EP1) in a direction D₁, and it continues into continuouschamber portion 12 _(CCP) so that direction D₁ thereby can becharacterized as clockwise in FIG. 10. Moreover, because of thecontinuous nature of portion 12 _(CCP), and further contributed by thepreferred dimension of its middle diameter, the wave will circulate in aconstructive manner in cell 12, thereby interrogating an ample amount ofdipolar gas in the process. And, such a result is achieved even throughthe path of the electromagnetic wave is omnidirectional, as opposed tothe bidirectional signal travel in various embodiments described above.Hence, the response of such interrogation will have an improved SNR asreceived by transceiver 16, via the exit extension portion 12 _(EP2),passage 12 _(P2), and receive antenna ANT_(RX).

From the above, various example embodiments provide a system generatingfrequency clock signals from atomic rotational quantum response in acell using bidirectional wave interrogation of the cell. Various exampleembodiments have various respects and/or benefits. For example, Q factorof the system response is improved (increased), thereby correspondinglyimproving Allan deviation. As still another benefit is that variousalterations have been provided, and still others may be ascertained.Accordingly, while various alternatives have been provided according tothe disclosed embodiments, still others are contemplated and yet othersmay be ascertained by one skilled in the art. Given the preceding, oneskilled in the art should further appreciate that while some embodimentshave been described in detail, various substitutions, modifications oralterations can be made to the descriptions set forth above withoutdeparting from the inventive scope, as is defined by the followingclaims.

From the above, various example embodiments provide a system generatingfrequency clock signals from atomic rotational quantum response in acell using a continuous chamber providing a circulating interrogation ofthe cell. Various example embodiments have various respects and/orbenefits. For example, SNR of the system response is improved. Asanother example, the system may perform well independent of pressure ofthe (e.g., dipolar) gas stored in the cell. As still another benefit isthat various alterations have been provided, and still others may beascertained. Accordingly, while various alternatives have been providedaccording to the disclosed embodiments, still others are contemplatedand yet others may be ascertained by one skilled in the art. Given thepreceding, one skilled in the art should further appreciate that whilesome embodiments have been described in detail, various substitutions,modifications or alterations can be made to the descriptions set forthabove without departing from the inventive scope, as is defined by thefollowing claims.

The invention claimed is:
 1. A clock apparatus, comprising: a gas cell,including a continuous path cavity including a sealed interior forproviding a signal waveguide; an apparatus for providing anelectromagnetic wave to travel along and circulate around the continuouspath cavity back toward and past a point of entry of the electromagneticwave in the continuous path cavity; a dipolar gas inside the sealedinterior of the continuous path cavity; and receiving apparatus fordetecting an amount of energy in the electromagnetic wave after theelectromagnetic wave passes through the dipolar gas.
 2. The clockapparatus of claim 1 wherein the receiving apparatus is further for:sweeping a frequency of an energy signal across a range of frequencies,wherein the range is provided to the electromagnetic wave; andresponsive to detecting a peak energy in the electromagnetic wave,maintaining a frequency of the electromagnetic wave at a frequencycorresponding to a frequency at which the peak energy occurred.
 3. Theclock apparatus of claim 2 wherein the peak energy includes a maximumamount of absorption.
 4. The clock apparatus of claim 2 wherein the peakenergy includes a minimum amount of transmission.
 5. The clock apparatusof claim 1 wherein the continuous path cavity includes a portion havinga circular planar cross-section.
 6. The clock apparatus of claim 5 andfurther comprising: an entrance portion in fluid communication with theportion having a circular planar cross-section; and an exit portion influid communication with the portion having a circular planarcross-section.
 7. The clock apparatus of claim 6: wherein the apparatusfor providing comprises a transmit antenna proximate the entranceportion; and wherein the receiving apparatus comprises a receive antennaproximate the exit portion.
 8. The clock apparatus of claim 5 whereinthe portion having a circular planar cross-section has a middle diameterproportional to a guided wavelength of the electromagnetic wave alongthe continuous path cavity.
 9. The clock apparatus of claim 5 whereinthe portion having a circular planar cross-section has a middle diameterproportional to a product of an integer times a guided wavelength of theelectromagnetic wave along the continuous path cavity, divided by pi.10. The clock apparatus of claim 1 wherein the gas cell is formed usingone or more layers in a semiconductor wafer.
 11. The clock apparatus ofclaim 1: wherein the gas cell is formed using one or more layers in asemiconductor wafer; and further including a transceiver forcommunicating a signal to the first apparatus, wherein the transceiveris embodied in an integrated circuit located in a fixed positionrelative the semiconductor wafer.
 12. The clock apparatus of claim 1wherein the apparatus is for providing an electromagnetic wave to travelin the cavity along the continuous path cavity and for circulating atleast 100 times around the continuous path cavity, each time back towardand past a point of entry of the electromagnetic wave in the continuouspath cavity.
 13. The clock apparatus of claim 1 wherein the apparatusfor providing an electromagnetic wave to travel along the continuouspath cavity is for circulating the electromagnetic wave to travel alongthe continuous path cavity in a constructive phase around the continuouspath cavity, back toward and past a point of entry of theelectromagnetic wave in the continuous path cavity.
 14. The clockapparatus of claim 13 wherein the travel along the continuous path isthrough a portion having a circular planar cross-section.
 15. The clockapparatus of claim 13 wherein the travel along the continuous path isthrough a portion having a trapezoidal cross-section taken perpendicularto a direction of the travel.
 16. The clock apparatus of claim 1:wherein the electromagnetic wave comprises a first electromagnetic waveto travel in a first direction along the continuous path cavity; andfurther comprising an apparatus for providing a second electromagneticwave to travel along the continuous path in a second direction oppositethe first direction.
 17. The clock apparatus of claim 16 wherein thereceiving apparatus for detecting an amount of energy in theelectromagnetic wave is responsive to an amount of absorption of atleast one of the first electromagnetic wave as the first electromagneticwave passes through the dipolar gas and the second electromagnetic waveas the second electromagnetic wave passes through the dipolar gas. 18.The clock apparatus of claim 1 wherein the continuous path cavitycomprises a perimeter proportional to a product of an integer times aguided wavelength of the electromagnetic wave along the continuous pathcavity.
 19. A method of operating a clock apparatus, the apparatuscomprising a gas cell, including a continuous path cavity including asealed interior for providing a signal waveguide, the method comprising:providing an electromagnetic wave to travel along and circulate aroundthe continuous path cavity back toward and past a point of entry of theelectromagnetic wave in the continuous path cavity; and detecting anamount of energy in the electromagnetic wave after the electromagneticwave passes through a dipolar gas inside the sealed interior of thecontinuous path cavity.
 20. The method of claim 19 and furthercomprising: sweeping a frequency of an energy signal across a range offrequencies, wherein the range is provided to the electromagnetic wave;and responsive to detecting a peak energy in the electromagnetic wave,maintaining a frequency of the electromagnetic wave at a frequencycorresponding to a frequency at which the peak energy occurred.